Cylindrical voice coils are commonly used on audio transducers such as cone drivers, dome tweeters, and microphone transducers. Typically, a cylindrical voice coil is suspended in a magnetic field, physically attached to a sound-generating diaphragm, and electrically connected to a signal source. The voice coil is usually a thin-walled tube having fine wire closely wrapped about the tube in a helical pattern. Glue is applied to secure the wire. A magnet structure provides an annular gap to receive the coil, with a radial magnetic field spanning across the gap to generate axial forces on the coil as a varying signal current flows through the coil. The conventional magnet structure is formed by a doughnut magnet having a front surface at a first polarity and a rear surface at the opposite polarity. An annular pole plate is attached to the front surface; a circular pole plate is attached to the rear surface, and includes an iron plug protruding forwardly through the doughnut hole to a position flush with the front surface of the front annular pole plate. Together, the plug and the front pole plate define an annular gap for receiving the coil. Magnetic field lines extend radially across the gap, with magnetic flux moving radially in only one direction.
A wire wound coil has several disadvantages. While other components of conventional cone transducers may otherwise be manufactured and assembled using highly automated processes, coil winding is more labor and skill intensive. Winding defects readily occur, often resulting in a significant number of rejected units that might not be discovered until after the product is completely assembled. To avoid excessive defects, coil winding machinery must operate at a limited speed. One type of failure mode common in wire coils is an imperfect wrap caused by a gap or overlap between adjacent wire loops. An overlapping wire may contact the magnet structure, resulting in unacceptable performance and eventual product failure during use.
Wire coil transducers have difficulty handling heat generated in the coil. During operation, current flowing through the coil generates heat that must be dissipated to prevent the coil from reaching excessive temperatures. The round wires employed in conventional voice coils have a relatively low surface area, and are therefore inefficient radiators. More important, the adhesive required to secure the wire to the core tube is vulnerable to failure at high temperatures. This failure can result in detachment of the wire. Even without detachment, thermal stresses may cause warpage of the entire voice coil, which may also result in catastrophic failure of the device.
It is believed that extensive efforts have been made throughout the audio industry to avoid the problems of wire coils by attempting to develop a more manufacturable alternative. Attempts may have been made to create flexible circuits, form them into cylindrical tubes, and provide numerous electrical connections at the junction between the two ends of the film to provide a helical conductor. Other attempts may have been made to deposit conductive material in a helical pattern on the interior or exterior surfaces of a thin walled tube. Apparently, none of these attempts has provided a suitable substitute for conventional voice coils.
Some sophisticated audio transducers employ magnet structures having more than one magnet, with the magnets being differently polarized. Such structures are disclosed in U.S. Pat. No. 4,903,308 to Paddock et al., U.S. patent application Ser. No. 07/916,038 to Paddock, filed July 17, 1992, U.S. patent application Ser. No. 07/730,172 to Paddock, filed Jul. 12, 1991, all of which are incorporated herein by reference.
To produce a transducer having differently polarized magnets, the magnets must be fully magnetized before assembly. Unfortunately, magnets tend to attract ferrous debris that may exist in the manufacturing environment. In addition, most magnets are formed of brittle ceramic material having potentially delicate edges that may break off, resulting in tiny particles that remain attracted. Thorough vacuuming is generally inadequate to remove all magnetically attracted particles.
If particles remain in the final assembly, they create unwanted noise during transducer operation, often at unacceptable levels. Accordingly, manufacturing must occur in a clean environment, and strict handling procedures must be employed to prevent shedding of magnet fragments. These measures increase manufacturing costs, and are not entirely effective.
Currently, inexpensive conventional magnet structures such as those employing a single doughnut magnet, as discussed above, may be fully assembled before magnetization. Special environmental and handling procedures are not required; before magnetizing, the assembly may be thoroughly and effectively vacuumed. The assembled structure is then placed in a strong magnetic field to magnetize the magnet. No further handling is necessary, and the delicate magnet is protected against damage by the surrounding structure.
Such post-assembly magnetism is difficult or impossible in transducers having multiple differently-polarized magnet structures. For example, existing two-way transducers having a cone woofer coaxially aligned with a dome tweeter generally employ separate magnet structures for the woofer and the tweeter. The need to carefully handle these pre-magnetized magnets increases the manufacturing costs as discussed above.